clocksource: add unregister function to disable unusable clocksources

On x86 the PIT might become an unusable clocksource. Add an unregister
function to provide a possibilty to remove the PIT from the list of
available clock sources.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --cc arch/x86/kernel/i8253.c
Simple merge

diff --cc include/linux/clocksource.h
Simple merge

diff --cc kernel/time/clocksource.c
Simple merge